/*
 * Derived from the RSA Data Security, Inc. MD5 Message-Digest Algorithm
 * and modified slightly to be functionally identical but condensed into control
 * structures.
 */

#include "md5.h"

#include <stdint.h>

/*
 * Constants defined by the MD5 algorithm
 */
#define A 0x67452301
#define B 0xefcdab89
#define C 0x98badcfe
#define D 0x10325476

static uint32_t S[] = {7,  12, 17, 22, 7,  12, 17, 22, 7,  12, 17, 22, 7,
                       12, 17, 22, 5,  9,  14, 20, 5,  9,  14, 20, 5,  9,
                       14, 20, 5,  9,  14, 20, 4,  11, 16, 23, 4,  11, 16,
                       23, 4,  11, 16, 23, 4,  11, 16, 23, 6,  10, 15, 21,
                       6,  10, 15, 21, 6,  10, 15, 21, 6,  10, 15, 21};

static uint32_t K[] = {
    0xd76aa478, 0xe8c7b756, 0x242070db, 0xc1bdceee, 0xf57c0faf, 0x4787c62a,
    0xa8304613, 0xfd469501, 0x698098d8, 0x8b44f7af, 0xffff5bb1, 0x895cd7be,
    0x6b901122, 0xfd987193, 0xa679438e, 0x49b40821, 0xf61e2562, 0xc040b340,
    0x265e5a51, 0xe9b6c7aa, 0xd62f105d, 0x02441453, 0xd8a1e681, 0xe7d3fbc8,
    0x21e1cde6, 0xc33707d6, 0xf4d50d87, 0x455a14ed, 0xa9e3e905, 0xfcefa3f8,
    0x676f02d9, 0x8d2a4c8a, 0xfffa3942, 0x8771f681, 0x6d9d6122, 0xfde5380c,
    0xa4beea44, 0x4bdecfa9, 0xf6bb4b60, 0xbebfbc70, 0x289b7ec6, 0xeaa127fa,
    0xd4ef3085, 0x04881d05, 0xd9d4d039, 0xe6db99e5, 0x1fa27cf8, 0xc4ac5665,
    0xf4292244, 0x432aff97, 0xab9423a7, 0xfc93a039, 0x655b59c3, 0x8f0ccc92,
    0xffeff47d, 0x85845dd1, 0x6fa87e4f, 0xfe2ce6e0, 0xa3014314, 0x4e0811a1,
    0xf7537e82, 0xbd3af235, 0x2ad7d2bb, 0xeb86d391};

/*
 * Padding used to make the size (in bits) of the input congruent to 448 mod 512
 */
static uint8_t PADDING[] = {
    0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};

/*
 * Bit-manipulation functions defined by the MD5 algorithm
 */
#define F(X, Y, Z) ((X & Y) | (~X & Z))
#define G(X, Y, Z) ((X & Z) | (Y & ~Z))
#define H(X, Y, Z) (X ^ Y ^ Z)
#define I(X, Y, Z) (Y ^ (X | ~Z))

/*
 * Rotates a 32-bit word left by n bits
 */
uint32_t rotateLeft(uint32_t x, uint32_t n)
{
    return (x << n) | (x >> (32 - n));
}

/*
 * Initialize a context
 */
void md5Init(MD5Context *ctx)
{
    ctx->size = (uint64_t)0;

    ctx->buffer[0] = (uint32_t)A;
    ctx->buffer[1] = (uint32_t)B;
    ctx->buffer[2] = (uint32_t)C;
    ctx->buffer[3] = (uint32_t)D;
}

/*
 * Add some amount of input to the context
 *
 * If the input fills out a block of 512 bits, apply the algorithm (md5Step)
 * and save the result in the buffer. Also updates the overall size.
 */
void md5Update(MD5Context *ctx, uint8_t *input_buffer, size_t input_len)
{
    uint32_t input[16];
    unsigned int offset = ctx->size % 64;
    ctx->size += (uint64_t)input_len;

    // Copy each byte in input_buffer into the next space in our context input
    for (unsigned int i = 0; i < input_len; ++i) {
        ctx->input[offset++] = (uint8_t) * (input_buffer + i);

        // If we've filled our context input, copy it into our local array input
        // then reset the offset to 0 and fill in a new buffer.
        // Every time we fill out a chunk, we run it through the algorithm
        // to enable some back and forth between cpu and i/o
        if (offset % 64 == 0) {
            for (unsigned int j = 0; j < 16; ++j) {
                // Convert to little-endian
                // The local variable `input` our 512-bit chunk separated into
                // 32-bit words we can use in calculations
                input[j] = (uint32_t)(ctx->input[(j * 4) + 3]) << 24 |
                           (uint32_t)(ctx->input[(j * 4) + 2]) << 16 |
                           (uint32_t)(ctx->input[(j * 4) + 1]) << 8 |
                           (uint32_t)(ctx->input[(j * 4)]);
            }
            md5Step(ctx->buffer, input);
            offset = 0;
        }
    }
}

/*
 * Pad the current input to get to 448 bytes, append the size in bits to the
 * very end, and save the result of the final iteration into digest.
 */
void md5Finalize(MD5Context *ctx)
{
    uint32_t input[16];
    unsigned int offset = ctx->size % 64;
    unsigned int padding_length =
        offset < 56 ? 56 - offset : (56 + 64) - offset;

    // Fill in the padding and undo the changes to size that resulted from the
    // update
    md5Update(ctx, PADDING, padding_length);
    ctx->size -= (uint64_t)padding_length;

    // Do a final update (internal to this function)
    // Last two 32-bit words are the two halves of the size (converted from
    // bytes to bits)
    for (unsigned int j = 0; j < 14; ++j) {
        input[j] = (uint32_t)(ctx->input[(j * 4) + 3]) << 24 |
                   (uint32_t)(ctx->input[(j * 4) + 2]) << 16 |
                   (uint32_t)(ctx->input[(j * 4) + 1]) << 8 |
                   (uint32_t)(ctx->input[(j * 4)]);
    }
    input[14] = (uint32_t)(ctx->size * 8);
    input[15] = (uint32_t)((ctx->size * 8) >> 32);

    md5Step(ctx->buffer, input);

    // Move the result into digest (convert from little-endian)
    for (unsigned int i = 0; i < 4; ++i) {
        ctx->digest[(i * 4) + 0] = (uint8_t)((ctx->buffer[i] & 0x000000FF));
        ctx->digest[(i * 4) + 1] =
            (uint8_t)((ctx->buffer[i] & 0x0000FF00) >> 8);
        ctx->digest[(i * 4) + 2] =
            (uint8_t)((ctx->buffer[i] & 0x00FF0000) >> 16);
        ctx->digest[(i * 4) + 3] =
            (uint8_t)((ctx->buffer[i] & 0xFF000000) >> 24);
    }
}

/*
 * Step on 512 bits of input with the main MD5 algorithm.
 */
void md5Step(uint32_t *buffer, uint32_t *input)
{
    uint32_t AA = buffer[0];
    uint32_t BB = buffer[1];
    uint32_t CC = buffer[2];
    uint32_t DD = buffer[3];

    uint32_t E;

    unsigned int j;

    for (unsigned int i = 0; i < 64; ++i) {
        switch (i / 16) {
        case 0:
            E = F(BB, CC, DD);
            j = i;
            break;
        case 1:
            E = G(BB, CC, DD);
            j = ((i * 5) + 1) % 16;
            break;
        case 2:
            E = H(BB, CC, DD);
            j = ((i * 3) + 5) % 16;
            break;
        default:
            E = I(BB, CC, DD);
            j = (i * 7) % 16;
            break;
        }

        uint32_t temp = DD;
        DD            = CC;
        CC            = BB;
        BB            = BB + rotateLeft(AA + E + K[i] + input[j], S[i]);
        AA            = temp;
    }

    buffer[0] += AA;
    buffer[1] += BB;
    buffer[2] += CC;
    buffer[3] += DD;
}

void md5Buf(uint8_t *input, int len, uint8_t *result)
{
    MD5Context ctx;
    md5Init(&ctx);
    md5Update(&ctx, input, len);
    md5Finalize(&ctx);
    memcpy(result, ctx.digest, 16);
}
